Health maintenance organizations (HMOs) strive to improve disease prevention and management while containing the costs of care. However, the mechanisms they employ to achieve these goals may impede access to care and reduce satisfaction with care among persons with complex medical "conditions such as cancer. When individuals have a choice between managed care and traditional fee-for-service (FFS) health insurance, their enrollment decisions may reflect actual or perceived differences among plans with regard to quality of care, access to needed services, and overall satisfaction. In order to better understand insurance enrollment decisions in Medicare beneficiaries with serious illness, we propose to examine rates and predictors of HMO disenrollment and enrollment in beneficiaries with and without cancer. Using a population-based cancer registry linked to Medicare enrollment records, we will estimate the rates of HMO disenrollment and enrollment in a cohort of beneficiaries with cancer and a matched cohort of cancer-free beneficiaries. We will also study potential predictors of enrollment decisions, including sociodemographic, health insurance market and disease characteristics. At a time when Medicare reforms are encouraging expansion of Medicare managed care, it is crucial that we understand how Medicare HMOs and traditional FFS coverage differ with regard to the enrollment of beneficiaries with complex, chronic and costly diseases, since enrollment disparities may reflect systematic differences in quality of care and may have ramifications for the long-term financial viability of the Medicare program. Our study will provide important information regarding HMO disenrollment and enrollment among Medicare beneficiaries and the effect of a serious illness on these decisions.